This invention relates to capacitive membrane ultrasound transducers (CMUTs) or other transducer. In particular, the invention relates to a multi-dimensional transducer.
Two-dimensional ultrasound transducers based on matrix array technology are currently limited to a few thousand elements or less. At operating frequencies equal to or below 3 MHz, this limitation is acceptable. For higher frequency probes (e.g., 5-10 MHz) intended for breast and small parts imaging, acceptable image quality cannot be achieved unless tens of thousands of acoustic elements are available for beamformation. However, tens of thousands of electrical connections from an array to an imaging system may not be possible or may be undesired. As a general rule, the element pitch must be less than or equal to one-half of a wavelength for phased arrays and one wavelength for linear arrays in order to achieve good image quality. Therefore, when the frequency is doubled, the wavelength is cut in half and four times as many elements are required for a transducer of the same size.
CMUTs may allow manufacture of arrays with tens of thousands of elements. CMUTs generate and receive ultrasound energy. An array of membranes with respective evacuated cavities between the membrane on the surface of a silicon wafer and the silicon substrate (e.g., cells) are fabricated on silicon wafers using semiconductor processing techniques. Electrodes are deposited on the membrane and the opposing face of the cavity under the membrane. These two electrodes form a capacitor. When the capacitor is charged electrically (or electrically biased), electrostatic forces pull the membrane toward the substrate electrode. In this state, changing the voltage on the capacitor modulates the electrostatic force on the membrane and causes the membrane to move up or down. In a reciprocal fashion, forcing the charged membrane to move up and down changes the voltage on the capacitor.
CMUTs offer many advantages over traditional ceramic transducers. For example, electrostatic transducers may be cheaper to manufacture, allow higher manufacturing yields, provide more size and shape options, use non-toxic materials, and have higher bandwidth. However, electrostatic transducers require a bias voltage for operation. The bias voltage in combination with any transmit voltage is limited to avoid collapse of the membrane. The electrostatic attraction of the membrane cannot exceed the restoring spring force of the membrane. Likewise, the dielectric breakdown of the gap between electrodes is usually avoided.
Another advantage is a reduction in the number of connections to an imaging system. Monolithically integrated receive electronics incorporating partial beamformation, data multiplexing, or data compression techniques offer a solution to the channel count problem for high frequency two-dimensional arrays, but only on the receive side. Transmit remains a difficulty since high voltage drivers for tens of thousands of channels cannot fit in the probe handle, and since an insufficient number of coaxial cables are available to route the transmit signals to the probe from the imaging system.